1. Field
The disclosure relates to a control circuit for a step-down DC-DC converter, a step-down converter, and a control method thereof. More particularly, it relates to a step-down DC-DC converter in which a main switching transistor includes an NMOS transistor, a control circuit for the step-down DC-DC converter and a control method of the step-down DC-DC converter.
2. Description of Related Art
In the technology disclosed in Japanese Laid-open Patent Publication No. 2002-084741, a series circuit including a resistor 103, a diode D101 and a speed-up capacitor C102 is connected in parallel with a voltage dividing resistor R101 at an output terminal side of a resistor voltage dividing circuit. At the stage when a terminal voltage of resistor R101 reaches or exceeds 1 Vf required for turning on the above diode R101 after start-up, the diode D101 is turned on to thereby start charging the speed-up capacitor C102. The charging voltage of the speed-up capacitor C102 at this time is at most equal to the difference between the terminal voltage of resistor R101 during a voltage stabilizing operation and the 1 Vf, so that the speed-up capacitor C102 is gradually charged even if its capacitance is small. As a result, a change in the voltage division ratio of the resistor voltage dividing circuit is also carried out gradually, thereby preventing output voltage overshoot.
Japanese Laid-open Patent Publication No. 2005-174351 discloses a technology for preventing overshooting and undershooting.
However, the technology described in Japanese Laid-open Patent Publication No. 2002-084741 requires the provision of one resistor element, two diodes and one capacitative element at the exterior, which inevitably increases circuit size. Also, in the event a main switching transistor is constituted of a NMOS transistor and a synchronous rectifier transistor is employed, the OFF timing of the main switching transistor is delayed. As a result, the main switching transistor and the synchronous rectifier transistor become conductive at the same time so that a through current flows therethrough. As a result, the main switching transistor and the synchronous rectifier transistor are likely to be damaged.